Olfactory function may be important for environmental and nutritional safety and enjoyment. Population-based epidemiological studies of olfaction are needed to understand the magnitude of the health burden, identify modifiable risk factors, and develop and test prevention and treatment strategies for olfactory impairment. However, measuring olfaction in large studies is challenging, requiring repeatable, efficient methods that can measure change over time. Two large cohort studies, the Epidemiology of Hearing Loss Study (EHLS) and the Beaver Dam Offspring Study (BOSS), included olfactory testing. In both studies, the San Diego Odor Identification Test (SDOIT) was used to measure olfaction. Subjects were asked to identify eight common household odors (such as coffee and chocolate). Olfactory impairment was defined as correctly identifying fewer than six out of eight odorants after two trials. The EHLS participants were age 53-95 years at the time of the first measurement (1998-2000), and participants in the BOSS were age 21-84 years. The prevalence of olfactory impairment in the EHLS was 25% overall, more common in men than women, and increased with age. Five years later olfaction was measured a second time and the majority (84%) of the EHLS participants were classified the same. Among those with impairment at the base line nearly one-third (31%) improved to unimpaired. This heterogeneity in olfactory impairment has unique implications for data analyses and predicting outcomes and associations. Preliminary data from the BOSS suggest the prevalence of olfactory impairment may be lower in younger generations. All these factors point to a continuing need for epidemiological studies of olfaction.